TransformerGraph clears graph when loop is introduced

[arneboe]

We found a special corner case that seems to clear the graph when introducing a loop in the TransformerGraph.

Test case to reproduce:

BOOST_AUTO_TEST_CASE(it_handles_loops_without_breaking)
{
    NodePtr root(TransformerGraph::create("root"));
    
    TransformerGraph::setTransformation(*root, "frame1", "frame2", Identity, Zero);
    TransformerGraph::setTransformation(*root, "frame3", "frame1", Identity, Zero);
    TransformerGraph::setTransformation(*root, "frame3", "frame2", Identity, Zero);

    BOOST_REQUIRE(TransformerGraph::hasFrame(*root, "frame1"));
    BOOST_REQUIRE(TransformerGraph::hasFrame(*root, "frame2"));
    BOOST_REQUIRE(TransformerGraph::hasFrame(*root, "frame3"));
}

[doudou]

Can you try #43 ?

It would be great if you could integrate your test, but make it actually test whether the resulting tree is valid.